As previously explained, in several reactor cores, sensors belonging to the RIC-type instrumentation system have been replaced by collectron-type sensors. Such reduction, which typically involves passing the number of sensors of the RIC system from 58 to 42, thus makes the number of collectrons fixed in the reactor core amount to 16. Hence, 16 guide tubes, which were normally monitored by mobile probes of the RIC system, are no longer of use when establishing a mapping deduced solely from the RIC sensors. Such a configuration results in a significant reduction in the number of instrumented channels monitored by the sensors of the RIC reference system, such sensors enabling to perform mappings that are normally required for checking the compliance of the reactor's core during start-up tests and for ensuring the periodic monitoring of the hot spot factors throughout the irradiation cycle.
Moreover, in the invention, interest is particularly given to the collectron-type detectors. Collectrons are the detectors located at a fixed elevation inside the nuclear reactor core, and which are able to supply datum on a non-stop basis. The most widespread collectrons are the Rhodium-type collectrons. The measurements performed are directly processed online by an integrated calculator or by a section calculator. The system's response time essentially depends upon the performances of said calculator which determines the calculation time. The operating principles of the collectrons are henceforth known and are available in various literature.
A major problem to be resolved when using the collectrons resides in the fact that the MUN uncertainty component increases significantly depending on the time passed inside the reactor and on the wear of the Rhodium emitter associated with the considered collectron.
In order to take account of such wear of the Rhodium emitter, a correction law has been established after seven years of experimentation in a powerful reactor: the application of such law to the signal supplied by a collectron upon completion of an operating t-duration enables to relocate the signal that said collectron would have emitted at the outset. Such law, called “sensitivity law”, is stated as follows:
                              S          ⁡                      (            t            )                          =                              S            ⁡                          (              0              )                                ×                                    (                              1                -                                                      Q                    ⁡                                          (                      t                      )                                                                            Q                    ∞                                                              )                        a                                              Relation        ⁢                                  ⁢        0            
With:                Q(t)=∫I(t′).dt′ and where I(t) is the gross current supplied by the detector to a t-instant. In practice, the initial signals supplied by the detector have had to be conditioned by deconvolution methods (in order regain lost time linked to the characteristics of the nuclear reactions in play) and by filtering (in order to reduce the noise induced by the deconvolution methods). The basic term should here be understood as “before correction of wear”.        S(0) is the initial sensitivity of the detector and Q∞ is its total available load.        The exponent a is a coefficient determined empirically as a result of experimentation.        
If the current supplied by a collectron at a given point in time of its irradiation is referred to as I(t), and the signal that should have had the same non-depleted collectron is referred to as I(0), then the sensitivity correction is performed according to the following relation:
                              I          ⁡                      (            0            )                          =                              I            ⁡                          (              t              )                                            S            ⁡                          (              t              )                                                          Relation        ⁢                                  ⁢        0        ⁢        bis            
A major consequence of the relation 0 is that, by implicating the integrated load, the application of said relation results in an accumulation of uncertainties on the debited current, whereby an increase in the overall error in accordance with the irradiation time.
Hence, this uncertainty, or error, estimated at 2% at the start of life, attains between 4.3% and 68% of wear and exceeds 8% at collectron life end for 80% of wear, such as represented in FIG. 3, which illustrates the sensitivity law and the MUN uncertainty component for a Rhodium collectron in accordance with the wear of the considered detector, as well as, for comparison purposes, the uncertainty component MUN for a RIC-type detector.
In comparison, it can be recalled that the MUN component of the RIC system is less than 2% and undergoes no increase during irradiation.